The mean camber line of an airfoil section is the curvature defined by a line halfway between the upper and lower surfaces of the airfoil section. The camber of an airfoil section affects air flow over the airfoil and therefore affects the lift generated by the airfoil.
It is known to regulate the loads acting on the blades of a wind turbine rotor with devices that modify the camber of the blades. Such devices include adjustable flaps, for example trailing edge flaps, leading edge flaps, ailerons, slats and Gurney flaps. In general, these flaps are supported by bearings that facilitate relative movement between the flaps and the body of the blade. In such hinged flaps, the bearings tend to be the main failure point due to mechanical wear and tear. Aside from wear and tear, a disadvantage of hinged flaps is that they often disrupt the otherwise continuous surface of the blade, which can cause aerodynamic noise and excessive drag, and reduce the efficiency of the wind turbine. A further disadvantage of hinged flaps is that the gap between the flap and the blade body may expose components inside the blade to environmental attack. For example, moisture or debris may enter the blade through the gap and damage the actuation mechanism of the flap, particularly if that water forms ice inside the blade.
Aside from hinged flaps, it is also known to modify the shape of blades by incorporating deformable parts into the structure of the blades. For example, WO2004/088130 discloses a wind turbine blade having a deformable trailing edge portion made from elastic material such as rubber. The trailing edge portion is configured to deform or flex without disrupting the continuity of the outer surface of the blade.
A deformable trailing edge portion of a wind turbine blade may be required to move or flex over a large range and at high frequencies. For example, known trailing edge portions may be required to flex at a frequency of approximately 1 Hz over a 20 degree range, and at a frequency of approximately 10 Hz over a 5 degree range. In many systems, to facilitate this level of flexing, the deformable trailing edge portion must be capable of extending and compressing by several millimeters. This large-range high-frequency flexing often causes fatigue in the deformable materials of known systems.
Aside from wind turbines, deformable portions are also known in aircraft wings. For example, US 2006/0145031 describes an aircraft wing formed from corrugated carbon fibre reinforced plastic (CFRP) plates. The wing extends from root to tip in a wing span or “spanwise” direction, and extends transversely from a leading edge to a trailing edge in a wing chord or “chordwise” direction. The corrugations in the CFRP plates increase the bending flexibility of the wing in the chordwise direction. The elastic action of the corrugations increases the compressive strength of the wing and prevents the wing from buckling when flexing. In one embodiment (FIG. 11), elastic material is impregnated into the concave “valley” portions of the corrugations. The elastic material fills the valleys and results in a smooth aerodynamic external surface of the wing. The elastic action of the elastic material also serves to increase the compressive strength of the wing in the corrugated region and hence permits a greater range of flexing of the wing in the corrugated region without buckling. In another embodiment (FIG. 13), a layer of elastic material is applied on top of the CFRP plates to achieve a smooth aerodynamic surface whilst enclosing air in the valleys of the corrugations. The elastic material and the compressibility of the air in the valleys together serve to increase the compressive strength of the wing in the corrugated region and allow the wing to flex over a greater range without buckling. In each of the embodiments described in US 2006/0145031, stiffness in the wing span direction is maximised by arranging the CFRP plates so that the reinforcing carbon fibres are aligned unidirectionally in the wing span direction.
Known flap systems are generally expensive, and may be relatively complicated to manufacture and/or attach to the blade. In addition, known systems often suffer from poor mechanical stability. Furthermore, most known systems rely on elastic materials such as rubber to provide flexibility. However, rubber generally has a short lifetime, especially when exposed to sunshine or extremes of weather. Consequently, known systems may have a short service life and may require frequent replacement.
Against this background, the present invention aims to provide an improved system for changing the profile that defines the camber of a wind turbine blade, which is relatively inexpensive and simple to manufacture, and which has a relatively long service life.